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Cancer starts when cells in the body begin to grow out of control. Cells in nearly any part of the body can become cancer, and can then spread to other areas of the body. To learn more about cancer and how it starts and spreads, see What Is Cancer?

Osteosarcoma (also called osteogenic sarcoma) is the most common type of cancer that starts in the bones. The cancer cells in these tumors look like early forms of bone cells that normally help make new bone tissue, but the bone tissue in an osteosarcoma is not as strong as that in normal bones.

Most osteosarcomas occur in children, teens, and young adults. Teens are the most commonly affected age group, but osteosarcoma can develop at any age. (For information about the differences between childhood cancers and adult cancers, see Cancer in Children.)

Where does osteosarcoma start?

In children, teens, and young adults, osteosarcoma usually starts in areas where the bone is growing quickly, such as near the ends of the leg or arm bones:

• Most tumors develop in the bones around the knee, either in the lower part of the thigh bone (distal femur) or the upper part of the shinbone (proximal tibia).
• The upper arm bone close to the shoulder (proximal humerus) is the next most common site.

Still, osteosarcoma can develop in any bone, including the bones of the pelvis (hips), shoulder, and jaw. This is especially true in older adults.

Subtypes of osteosarcoma

Based on how the cancer cells look under the microscope, osteosarcomas can be classified as high grade, intermediate grade, or low grade. The grade of the tumor tells doctors how likely it is that the cancer will grow quickly and spread to other parts of the body.

High-grade osteosarcomas

These are the fastest growing types of osteosarcoma. When seen with a microscope, they do not look like normal bone, and many of the cancer cells are in the process of dividing into new cells. Most osteosarcomas that occur in children and teens are high grade. There are many types of high-grade osteosarcomas (although the first 3 are the most common).

• Osteoblastic
• Chondroblastic
• Fibroblastic
• Small cell
• Telangiectatic
• High-grade surface (juxtacortical high grade)

Other high-grade osteosarcomas include:

• Pagetoid: a tumor that develops in someone with Paget disease of the bone
• Extraskeletal: a tumor that starts in a part of the body other than a bone (but still makes bone tissue)
• Post-radiation: a tumor that starts in a bone that had once been exposed to radiation

Intermediate-grade osteosarcomas

These uncommon tumors fall between high-grade and low-grade osteosarcomas. (They are usually treated the same way as low-grade osteosarcomas.)

• Periosteal (juxtacortical intermediate grade)

Low-grade osteosarcomas

These are the slowest-growing osteosarcomas. The tumors look more like normal bone and have few dividing cells when seen with a microscope.

• Parosteal (juxtacortical low grade)
• Intramedullary or intraosseous well differentiated (low-grade central)

The grade of the tumor plays a role in determining its stage and the type of treatment used. For more on staging, see Osteosarcoma Stages.

Other types of bone tumors

Several other types of tumors can start in the bones.

Malignant (cancerous) bone tumors

Ewing tumors (Ewing sarcomas) are the second most common bone cancer in children. They are described in Ewing Family of Tumors.

Most other types of bone cancers are usually found in adults and are uncommon in children. These include:

• Chondrosarcoma (cancer that develops from cartilage)
• Undifferentiated pleomorphic sarcoma (UPS) of bone, previously known as malignant fibrous histiocytoma (MFH) of bone
• Fibrosarcoma of bone
• Chordoma
• Malignant giant cell tumor of bone

For more information on these cancers, see Bone Cancer in Adults.

Many types of cancer that start in other organs of the body, especially cancers in adults, can spread to the bones. These are sometimes referred to as metastatic bone cancers, but they are not true bone cancers. For example, prostate cancer that spreads to the bones is still prostate cancer and is treated like prostate cancer. For more information, see Bone Metastasis.

Benign (non-cancerous) bone tumors

Not all bone tumors are cancer. Benign bone tumors do not spread to other parts of the body. They are usually not life threatening, and surgery can often remove them completely. There are many types of benign bone tumors, including:

• Osteoma
• Chondroma
• Osteochondroma
• Eosinophilic granuloma of bone
• Non-ossifying fibroma
• Enchondroma
• Benign giant cell tumor of bone
• Lymphangioma